Impedance matching network



Oct. 27, 1953 IMPEDANCE W. H. EPPERSON IMPEDANCE MATCHING NETWORK Filed May 15. 1951 MPEDANCE Patented Oct. 27, 1953 UNITED STATES OFFICE 2.6 62. IMPEDANCE.- MATCHING NETWORK William H. Epperson, coral Gables, Fla., assignor to Aeronautical Communications Equipment,-

Inc Miami, Fla acorporationof Elorida" Application May 15, 1951, Serial-No. 226,517

'7 Glaims. (61 333-32) This invention relates-to impedance-matching, networks and, more particularly, to-apparatus having a single unified control of the variable reactive elementsv by means of which the impedances are matched.

Impedance matching networks are known wherein the controls of the variable reactive ele ments in the impedance matching arrangement are so geared together that one element goes through its range of variation several times while another element goes through its range only once. Such an arrangement permits all the con trols to be actuated by rotation of a single drive shaft and is particularly useful in automatic antenna tuning systems such as are employed with aircraft radio transmitters.

The present, invention primarily resides in a network capable of matching av relatively con stant impedance to a variable impedance throughout, a broad range of frequencies. network has a first variable capacitive voltage divider connected between the relatively constant impedance and ground potential. A variable inductor is connected between an intermediate capacitance pointjon the first voltage divider and the high impedance side of a second variable capacitive voltage divider. The second voltage divider has its low impedance side at ground potential and has an intermediate capacitance point connected, to. the variable impedance. The-reactances of the inductor and 013 the capacitive voltage dividers are simultaneously variable at difierentspeeds under a unified; control to effect the impedance match. It is, of

course, through the network and, accordingly, the rela tively constant impedance may be either a source or a load.

In order that the invention may be moreclearly understood, it will now be described in detail with reference to the accompanying drawing wherein:

Fig. 1 is a schematic circuit diagram ofthe novel impedance matching network;

Fig. 2 is a schematic circuit diagram of an embodiment of the invention particularly adapted for matching the impedance of a transmission line to an antenna; and

Fig. 3 is a schemtic drawing of the mechanical form of the impedance matching network shown in Fig. 2.

Referring now to Fig. l, a substantially constant impedance l i is coupled to an independent- 1y vari b im nc Z y ea s of an impedante m hi n o k om i ing a va able 9%.-

immaterial which way power flows pacitiveyoltage divider l3, avariable inductor M, and a second. variable capacitive voltage divider 15 connected in. a Pi .or L configuration The adjustable portions of reactive elements it, t l and I't'yare controlled by shafts It, I! and 1 8 respectively. A unified control member it, which; is here shown as a manual knob, is mounted ore shaft, IQ for the purpose of directly controlling; the varaible capacitive voltage divider It. A. pinion 2! fixed to shaft 1 6 meshes with a gear 22 attached to shaft II. A pinion 23 on shaft ll" likewise engages gear 24 on shaft 18. This gearing arrangement permits variation of all three variable reactive elements l3, l4, [5 at different speeds by rotation of member 19.

A unified control of this type is particularly adapted to be motor-driven in response to an impedance sensitive circuit which produces an error signal in the presence of a mismatch. This error signal is reduced to zero when the irnapedance matching networkis correctly adjusted- Such automatic systems have found application in tuningv an antenna to a radio transmitter and per se form no part ofithe present invention.

Capacitive voltage divider l3 may be formed with conventional air capacitors 25 and 26 connected in series" between impedance H and. ground potential with their rotors mechanically coupled 180 out of phase. The inductor it has. one of its terminals connected to the common junction of. capacitors 25 and 26. The capacitive voltage divider I5 may be generally similar tovoltage divider I3 may comprise variable capacitors'2'l and 28 connected in series between the second terminal of inductor l4 and ground po tential. The common junction between capacitors'z'l and 28, i. 'e. the intermediate capacitance point on voltage divider I5, is connected to impedance l2. Impedances H and i2 may be source and load impedances, respectively, or vice versa.

In operation of the impedance matching net work, capacitive voltage divider [3 passes through the complete cycle of its reactance values from minimum" to maximum and again to minimum a. large number of times while the inductor is adjusted once over its range of inductance. Capacitive voltage divider it passes through its range of reactance values an intermediate number of times corresponding approximately to the square root of the cycles performed by voltage divider 13. The precision of adjustment is sub stantially proportional to the speed ratio of the different reactive elements and the highest practical value is usually selected. It is to be obgi e tha its Wi e. ra e of imp a athing performed by the network is largely due to the differential action of the capacitive voltage dividers l3 and 1". When series capacitor 25 or 21 is maximum, the associated shunt capacitor 25 or 28 is at a minimum. Impedance ll tends to be partially decoupled from inductor M by capacitor 25 at the same time that the shunting effect or capacitor 2t is at a maximum (condition for minimum coupling to the inductor l4).

2 and 3 disclose a form of the impedance matching network. which is particularly suitable for matching a radio transmission line 3| to an antenna 32. Capacitive voltage divider l3 here takes the form of a differential butterfly type of air capacitor consisting of four stator plate assemblies and 36 each covering a sector of slightly less than 9%" and a rotor Ell having two oppositely disposed plate assemblies each also covering a sector or" slightly less than 90. Opposite stator assemblies are connected together electrically, stators 3 being attached to the transmission. line while stators 35 and 3?; are grounded. The rotor 33 is insulated from ground and connected by a wiper arm 38 to one end of the inductor It is to be noted that with each rotation of shaiu is the capacitive voltage divider it passes through two complete variations of its capacitance. also the rotor assemblies are nice. nioally balanc-.d with the result that the butterfly type of rotor can be turned at half the speed of the voltage divider it in Fig. l for the same number of capacitance changes and with less vibration.

A suitable construction of inductor i is shown in Fig. 3 wherein cylindrical coil form til is car-- ried on shaft ii. A helical coil is wound in a threaded groove in form has ends electrically connected to slip rings ii and t2 concentric with and insulated from shaft ii. A

ut sliding contact between slip .it on which is slidably mounted roller wiper Roller wiper iii is adapted to males rolling electrical contact between a turn of coil til and shalt while sliding axially along shaft it coil form Eli is rotated shaft i'i. is construction is well known and not per form a part of the invention. A. varioineter would be a satisfactory substitute for roller coil for the fact ..nductance r nge is more limiter. A. brush til conta "rig slip ring il serves to connect inductor l to capacitive voltage divider l The capaci 've voltage divider it comprise two stator plate assemblies ll and and a rotor plate assembly each assembly including sector of slightly less than 120. Stator assembly l? is connected by way of brush it to inductor it while stator assembly ll; is placed at ground potential. Rotor assembly ts is connected by 1" eons of a brush ill to e antenna It is to be noted that the rote. assembly is turned the capacitance of voltage divider id is varied be tween the output end of the variable inductor i lthe antenna also is varied between. an-

and ground. vi bile one of these capacitances diminishing the other is increasing and vice versa. This arra. gement provides a continuous and smooth transfer of the impedance etworl: from. Pi circuit to an L. cir- The sectors covered by the stator and rotor plate assemblies in capacitive voltage divider are chosen to provide the maximum capacitance between the rotor and each oi the stators and between the rotor and both of the stators and at the same time to achieve the lowest minimum distributed capacitance between the rotor and one stator. It is believed that these conditions are most nearly met when the sectors of all the plate assemblies are slightly less than In one practical embodiment of the invention a transmission line having a characteristic impedance of 52 chi. s was matched to an antenna over a wide range of frequencies where the resistive component varied from about 2 ohms to about 25,000 ohms and with its normal reactive variation. In this particular impedance matching network, the three variable reactive elements were so geared together that while the variable inductor is was adjusted from minimum to maximum inductance, the capacitive voltage divider l5 passed through 15 cycles and the capacitive voltage divider 13 passed through 690 cycles. The speed ratio of voltage divider it differed from its theoretically optimum value in order to compensate in a piactic? manner for the porn linear variation of indoor of the The inductor comprised 30 turns of w' a made one complete change of inductance for SO revolutions of its shaft i'i. Aecorr'i ly, the ratio between gears 2i and 22 was 10:1 the ratio between gears 23 and was 2:1. It standing wave ratio of 1.2 to l or better was attained under these conditions, A closer impedance match could be achieved if a higher speed ratio were employed between the three reactive elements.

I claim:

1. [in impedance matching network for matching a first impedance to a second impedance comprising a first variable capacitive voltage divider connected between the first impedance and ground. potential, a two-terminal variable inductor having one terminal connected to an intermediate capacitance point on said first voltage di vider, a second variable capacitive voltage divider connected between. the second terminal of said inductor and ground potential, said second irnpedanee being connectec to an intermediate capacitance point on said second voltage divider, and means for simultaneously varying the reactances of said inductor and capacitive voltage dividers at dilierent speeds to effect an impedance match.

2. An impedance matching network for matching a substantially constant impedance to an independently variable impedance comprising a first variable capacitive voltage divider connected between the constant impedance and ground potential, a two-terminal variable inductor having one terminal connected to an intermediate capacitance point on said first voltage divider, a second variable capacitive voltage divider connected between the second terminal of said inductor and ground potential, said variable impedance being connected to an intermediate capacitance point on said second voltage divider, and means for simultaneously varying the reactance of said first capacitive voltage divider at a greater speed than the reactance of said sec- 0nd capacitive voltage divider and the reactance of said second capacitive voltage divider at a greater speed than the reactance of said inductor.

3. An impedance matching network for inatching a first impedance to a second impedance comprising a variable input capacitor having an assembly of rotor plates meshable with two insulated assemblies of stator plates, one stator assembly being connected to ground potential and the other stator assembly being connected to the first impedance, a two-terminal variable inductor having one terminal connected to said rotor assembly, a variable output capacitor having an assembly of rotor plates meshable with two insulated assemblies of stator plates, one stator assembly being connected to the second terminal of said inductor and the other stator assembly being connected to ground potential, the second impedance being connected to said rotor assembly, and means for simultaneously varying the reactances of said inductor and said capacitors at different speeds to efiect an impedance match.

4. An impedance matching network for matching a substantially constant impedance to an independently variable impedance comprising a variable input capacitor having an assembly of rotor plates meshable with two insulated assemblies of stator plates, one stator assembly being connected to ground potential and the other stator assembly being connected to the constant impedance, a two-terminal variable inductor having one terminal connected to said rotor assembly, a variable output capacitor having an assembly of rotor plates meshable with two insulated assemblies of stator plates, one stator assembly being connected to the second terminal of said inductor and the other stator assembly being connected to ground potential, the variable impedance being connected to said rotor assembly, and means for simultaneously varying the reactance of said inductor slowly, said output capacitor more quickly and said input capacitor the quickest to effect an impedance match.

5. An impedance matching network according to claim 4 wherein the rotor and stator plate assemblies of the output capacitor cover sectors of slightly less than 120.

6. An impedance matching network according to claim 4 wherein the stator plate assemblies of the input capacitor cover sectors of slightly less than and the rotor plate assembly of said input capacitor covers two oppositely disposed sectors of slightly less than 90.

7. An impedance matching network according to claim 6 wherein the rotor and stator plate assemblies of the output capacitor cover sectors of slightly less than WILLIAM H. EPPERSO-N.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,498,078 Harrison Feb. 21, 1950 FOREIGN PATENTS Number Country Date 849,954 France Aug. 28, 1939 514,298 Great Britain Nov. 6. 1939 

